It is well known that considerable difficulties may arise in the production of castings from metals, such as aluminum and its alloys, magnesium and its alloys, etc., due to the incidents of defects associated with dissolved and dispersed impurities. The main dissolved impurities are hydrogen and alkali or alkaline earth metals, e.g. lithium, sodium, calcium, etc., while the dispersed impurities are typically solid inclusions, such as oxides, carbides, borides, nitrides, etc.
The injection of an inert gas or inert-reactive gas mixture into molten metal is a commonly used technique for the removal of above contaminants. Such systems are described in Szekel, U.S. Pat. No. 3,743,263, Withers et al, U.S. Pat. No. 4,634,105, etc. These prior systems are based upon injecting gas in the form of small discrete bubbles throughout the melt. Hydrogen is removed from the melt by desorption into the gas bubbles, while the solid inclusions are lifted into the dross layer by flotation.
In order to maximize the metallurgical efficiency of any gas/liquid injection process, it is essential to obtain good coupling between the liquid phase and the gaseous phase. In particular, the generation of large interfacial area between the gas and the liquid is absolutely necessary and to maximize mass transfer and improve the kinetics of the reaction. In the case of high surface tension liquid metals, such as liquid aluminum, the energy required to generate this large interfacial area is best provided by generating high shear forces, e.g. by means of rotary gas injectors such as that described in U.S. Pat. No. 4,634,105.
However, the generation of a large gas/liquid interfacial area is not the only important process requirement to maximize overall process efficiency. Good bulk liquid mixing or circulation is also essential. For instance, it has been clearly demonstrated that when gas is injected into a molten metal bath through stationary lances, the lack of effective metal circulation is one of the main factors limiting the efficiency of impurities removal. This is because large bubbles plumes are relatively ineffective in promoting bulk metal circulation. Gas momentum is negligible and circulation is driven exclusively by liquid entrainment into the rising bubble plume.
The melting furnaces used in the aluminum industry are typically in the form of very large shallow baths. Thus, such baths typically have liquid metal depths of no more than about 1.5 meters, but may have very large horizontal areas since furnaces having capacities in the range of 30-150 tons are now commonplace. It will be evident that stationary lances are most inefficient for these large shallow baths, since the residence time of the gas in the melt is very short and the power that can be introduced for stirring via the buoyant energy of the gas bubbles is extremely limited.
Another major factor in the treatment of molten metal with gases is the rate of surface turbulence. It is well established that surface turbulence must be minimized in order to avoid large quantities of dross being formed, as well as to minimize oxide inclusion formation and hydrogen absorption through the surface of the bath. Furthermore, when chlorine is used as a reactive gas, serious environmental problems are created when large quantities of chlorine are not reacted with the metal bath and are released from the surface of the melt.
Rotary gas injectors, such as that described in U.S. Pat. No. 4,634,105, work very well in small furnaces of less than about 30 tonnes capacity, but they present major practical problems when used in association with large furnaces. Thus, a large number of rotors must be mounted across the furnace and the moving parts and drive mechanisms associated with such rotors are most incompatible with the environment of metal melting furnaces. The initial costs of such systems are high and there are also very high maintenance costs.
Another system that works quite well for mixing gases with molten metals is an in-line fully mixed reactor, designed with one or several rotors/baffles combinations to subdivide the treatment gas into small individual bubbles and create strong mixing currents within the liquid bath. While such a system in highly effective for treating molten metal being moved through a conduit, it is not a practical answer for the treatment of large, shallow, static molten metal baths found in melting furnaces.
Yet another system for the treatment of molten metal with gases is a combination of a mechanical centrifugal circulating pump and a static gas injection lance located inside the enclosed outlet section of the pump. Such system is described in U.S. Pat. No. 4,169,584. However, it has the disadvantage of requiring a circulating system external of a large molten metal bath and it also has the high initial cost and maintenance problems associated with a mechanical rotary system for circulating molten metal.
It is also known to inject a reactive gas mixture into liquid metals using porous plugs mounted in the base or refractory wall of a furnace. Although such porous refractory elements give satisfactory results when used with liquid of low surface tension, their efficiency for the generation of small gas bubbles is rather poor when in contact with liquid metals of high surface tension, liquid aluminum or steel. Also, partly due to the high back pressure necessary by the fine porosity of these elements (in order to avoid liquid metal penetration into the refractory pores) it is very difficult to provide gas tight connections and interfaces between the porous element and the remainder of the refractory in the furnace. This results in gas diffusion between the steel structure and the refractory lining, causing internal gas leaks. These gas leaks can be extremely dangerous when reactive gas fluxing with chlorine or other reactive compounds is used. Moreover, the fixed porous refractory element does not address the problem of poor bulk liquid mixing and excessive surface turbulence.
U.K. Patent Application 2,054,396, published Feb. 18, 1981, describes a technique where the fluxing gas is sucked in through a pipe or several pipes by a reduced metal pressure produced by means of a standard venturi design. This technique requires a very small gas injection opening in the order of a few millimeters which is not practical for industrial application. The venturi system also requires a molten metal pump.
It is the object of the present invention to provide an improved gas injection system for molten metal which can, without moving parts, efficiently disperse small gas bubbles through a large molten metal bath.